1. Field of the Invention
The invention relates to a method for monitoring a radio transmission link between at least one radio transmission device and at least one radio reception device, and to a radio transmission system, particularly for carrying out such a method. The invention also relates to a radio reception apparatus for use in such a radio transmission system.
2. Description of the Related Art
Radio communication systems, particularly mobile radio systems, have in the meantime reached a high technological standard and are becoming more and more widespread. One important component which determines the performance of radio communication systems is antennas. To be able to ensure that a radio communication system is operable, the individual modules, particularly the antennas in the radio communication system, have to be monitored. Antennas are extremely difficult to monitor, however, because they are generally positioned in places which are difficult to access. Radio communication system operators therefore have an urgent requirement to be able to establish, simply and reliably, whether the antennas or the supply cables from the respective radio communication system to the corresponding antenna are briefly or permanently faulty. For example, an antenna which has been damaged by a storm or by corrosion results in considerable deterioration in the transmission quality of the airway situated in between.
The invention is therefore based on the object of providing a method, a radio transmission system and a radio reception apparatus which can be used for simple and reliable monitoring of a radio transmission link between at least one transmission antenna and at least one reception antenna.
The invention makes use of the fact that radio communication systems have a plurality of radio transmission devices and radio reception devices with dedicated transmission and reception antennas, so that, between a transmission antenna and a reception antenna, a closed RF path is formed from the transmission antenna to the reception antenna. This means that a signal emitted by a transmission antenna is received at a reception antenna and can be evaluated by the reception device associated with the latter.
To monitor a radio transmission link between at least one radio transmission device and at least one radio reception device, the at least one radio reception device is first tuned to the transmission frequency of the at least one radio transmission device. The at least one radio transmission device transmits a predetermined test signal during a predetermined time period. At this juncture, it should in fact be pointed out that the test signal can, for example, be a signal with a constant signal level or a signal with predetermined, for example discrete, level changes. The test signal transmitted by the at least one radio transmission device is recorded and evaluated by the at least one radio reception device during a predetermined time period. The predetermined time period can be chosen such that the test signal can be evaluated reliably in order that it may be possible to filter out random attenuation fluctuations on the radio transmission link, caused by a bird sitting on an antenna, for example. Each evaluated test signal is then stored in the at least one radio reception device for subsequent test purposes. Storage of the test signal as a reference signal ends the so-called initialisation phase of the radio communication system.
During a test phase of predetermined duration, at least one radio transmission device transmits the predetermined test signal. In the at least one radio reception device, the test signal emitted by the at least one radio transmission device is compared with the stored reference signal during the test phase and is evaluated. If the evaluation shows that the test signal has changed beyond a predetermined extent with respect to the stored reference signal, an error signal is produced which signals that the radio transmission link is faulty. This will generally mean antenna damage. However, it should be pointed out that this method does not enable the location of the fault within the radio transmission link to be determined, but only allows reliable ascertainment of whether or not the radio transmission link is faulty. In this case, the duration of the test phase can again be chosen so that short-term random attenuation changes on the radio transmission link are not taken into account. Otherwise, for example, a passing bird or aeroplane could cause incorrect interpretation of the state of the radio transmission link.
The method makes it possible, therefore, to monitor the radio transmission link between a radio transmission device and a plurality of radio reception devices all tuned to the transmission frequency of the single radio transmission device. It is also possible to monitor radio transmission links between a plurality of radio transmission devices and a single radio reception device. For this, the radio reception device must be tuned to the respective transmission frequencies of the individual radio transmission devices, and a reference signal from the respective radio transmission device must be stored in the radio reception device in each case.
So that the at least one radio reception device can derive a reference signal from the received test signal, at the start of the initialisation phase, the at least one radio transmission device transmits its operating mode and the test signal type to the at least one radio reception device. The radio transmission device can operate in burst or continuous mode, for example.
First a reference level and then only relative level changes with respect to the reference level of the test signal to be transmitted are transmitted to the at least one radio reception device.
In addition, the at least one radio reception device stores a tolerance range which defines the upper and the lower level limit within which the received test signal is permissibly situated in order to be evaluated as a reference signal. If, at least intermittently, the test signal is situated within the predetermined tolerance range during the predetermined time period, the recorded and evaluated test signal is stored as a reference signal. Otherwise, the initialisation phase must be repeated.
A critical factor for reliable and operationally correct monitoring of the radio transmission link is, amongst other things, the stability of the transmission signal, which is similarly also taken into account in the chosen tolerance range.
In order to be able to detect natural ageing phenomena and temperature fluctuations, the initialisation phase is repeated at predetermined instants, so that corrected reference signals can be formed. Expediently, the reference signals from each repeated initialisation phase are stored so that it is thus possible to obtain an indication about long-term attenuation changes on the monitored radio transmission link.
The invention provides a radio transmission system which comprises at least one radio transmission device, designed for transmitting a predetermined test signal, and at least one radio reception device. The radio reception device has at least one device which can be tuned to the transmission frequency of at least one radio transmission device, a device for recording and evaluating the test signal from the at least one radio transmission device during a predetermined time period, a device for storing at least one test signal as a reference signal, a device for comparing and evaluating the test signal received from the at least one radio transmission device with the corresponding reference signal during a test phase of predetermined duration, and a device for producing an error signal. The error signal is produced only if the comparison and evaluation device signals that the test signal has changed beyond a predetermined extent with respect to the stored reference signal.